The accurate Alternating Current (AC) measurement of electrical power at various points of a power grid is becoming more and more important and, at the same time, is getting more and more difficult. The old power distribution model of a few, large power generating stations and a multitude of relatively linear loads is being replaced by a newer model containing a multitude of smaller, and to some degree unpredictable power sources, as well as a multitude of not always linear and often smart (essentially also unpredictable) loads. This change deteriorates power quality and makes AC measurements, grid management and troubleshooting more difficult. Also, the increasing cost of electrical power makes precise calculation of delivered energy and monitoring of power quality important.
There are three main categories of AC power measurement systems: The highest level of accuracy systems, used typically by the Standard and Calibration Laboratories, are developed to reference measurement to the National Standards. These are typically unique installations, not covered by specific regulatory requirements. The next category is high precision AC power measurement systems. In the important case of AC power measurement instruments, usually referred to as Power Analyzers, these would be units meeting the requirements of standards, such as for example International Standard IEC 61000-4-30 “Electromagnetic Compatibility: Part 4—30 Testing and Measurement Techniques—Power Quality Measurement Methods” which relates to Class A measurement methods. These are used where precise measurements are necessary, for example for contractual applications and disputes, verifying compliance with standards, etc. Two different Class A instruments, when measuring the same quantities, should produce matching results within the specified uncertainty for that parameter. The third main category of the AC power measurement system is general use instruments. Generally, it is recommended that this group reflect measurement methods and intervals of Class A instruments, with lower precision and processing requirements. It is then classified as Class S. Other instruments including legacy installations, whose operation doesn't reflect methods of Class A, but still meet key accuracy requirements, are summarily called Class B. Irrespective of the class of the AC power measurements they require determination of the voltage, current, frequency, phase, and relative timing of the single or multiple phases of the power system in order to perform the measurements.
The whole measurement chain of electrical quantity for power analysis consists of measurement transducer, measurement unit and evaluation unit (as is defined in the ICE 61000-4-30 standard). The measurement transducer converts the input quantity to a level and a kind suitable for the measurement unit and typically has some other functionality, for example signal isolation or overload protection. For example, the measurement transducer may reduce a power line voltage of hundreds of kilovolts to tens of volts. The measurement unit then converts the input quantity, typically to a digital form, suitable for evaluation. Then the evaluation unit, which is typically some form of a computing device, receives and combines data streams from different input channels including for example the output of the measurement unit and a reference unit, and does the required calculations to produce results. Test results can be: recorded, aggregated, automatically evaluated in the real time, displayed on the instrument screen, used to generate alarms, placed in system logs, and send out for external evaluation and storage, etc.
Generally, AC electrical measurements are used in a wide variety of applications and may be performed for a variety of electrical quantities including voltage, current, capacitance, impedance, resistance etc. These tests and measurements include those relating to designing, evaluating, maintaining and servicing electrical circuits and equipment from high voltage electrical transmission lines operating at hundreds of kilovolts (kV) and kiloamps (kA) to industrial/medical/residential electrical and lighting, typically 400V/240V/100V and 30/15 A, to a wide variety of industrial/scientific/medical/consumer electrical and electronic devices.
Within a variety of applications and test equipment systems the measurement transducer is often a toroidal transformer. These allow for the measurement system to measure the required parameter(s) with the measurement system electrically isolated from the electrical system being measured. Further, toroidal forms of the core of the transformer provide best magnetic performance of the core, providing low magnetic reluctance, good uniformity of the magnetic field and low flux leakage, resulting in the best electrical parameters of the transformer. In general, the toroidal form of the core of the transformer is an accepted standard for meteorological applications.
However, with the continued drive for improved accuracy in calibration, standards, and measurements on circuits and components operating at hundreds of kiloVolts, thousands of Amps, with resistances into Gigaohms accuracies of parts per million is being replaced by parts per billion. At the same time as discussed supra such measurements are being performed upon, for example, electrical power distribution systems at various points of a power grid with a variety of generators, distribution systems, etc. with unknown or variable characteristics. On the other hand, even the best toroidal core transformers still have three basic limitations, affecting performance of the transducer, namely saturation of the core, finite value of the permeability and finite width of the hysteresis loop. Each one affects operation of the transformer and may limit overall accuracy of the resulting transducer. The first and most obvious way to improve performance of the measurement transformer is to use highest permeability, lowest losses (narrowest hysteresis loop) magnetic materials for the core.
Next, the inventors have established a measurement and correction methodology for AC current transducers designing multi-core, multi-stage transformers compensating effects of finite, changing burden. Similarly, DC compensation was introduced to improve AC operation of the measurement transformer in the presence of the DC components magnetizing the transformer core. Beneficially, such measurement and correction methodologies provide instrument designers with multiple options ranging from low cost alarms through to higher cost automated correction hardware software and firmware based circuits.
Such measurement and correction methodologies would beneficially allow such devices according to some embodiments of the device to achieve performance approaching that of reference measurements operating in laboratory conditions. It would be further beneficial if the same principles provide power utilities, independent electricity producers, electrical engineers and technicians, and others requiring accurate measurements of power systems with a field deployable power system measurement devices providing up to Class A type performance but in rugged devices of reduced cost and complexity.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.